1. Field or the Invention
The present invention pertains to instruments to depth measurement or loosely laid pavement mixtures and more particularly to depth measurement instruments used to measure a hot bituminous pavement layer while paving before compaction to determine whether the proper depth or pavement has been set in place.
2. Related Prior Art
Presently, measuring the mat thickness of a bituminous paving layer before compaction is normally done by a very simple portable device. This device usually consists of a one quarter inch rod that has an adjustable washer. The rod with washer fixed in place is inserted into the recently laid bituminous pavement mixture of aggregates and asphalt cement. The rod may be several feet long so that it is easy to handle and can measure significant depths of pavement. The rod may also be threaded so that a lock nut can be used in conjunction with the washer. The washer may be fixed to the nut so that it will not slide when the nut is screwed in place. In most instances, the washer is used in conjunction with a set screw. This arrangement avoids the problem of asphalt tar filling in the threads and preventing movement to the nut.
A ruler normally is used to measure the distance from the end of the rod to the washer to set the desired mat thickness. The washer is then locked down on the rod usually by means of a set screw. The person measuring the mat or bituminous paving layer thickness then walks where the mat has just been laid and shoves the rod into the bituminous paving layer. The rod is pushed into the bituminous paving to see if the rod goes to or exceeds the distance of the washer from the end of the rod. If the washer does not touch the mat, the thickness of the mat is increased. If the bituminous paving exceeds the depth of the washer, the thickness is decreased.
One problem associated with such a device comes from the composition of the paving material itself. This problem occurs primarily because of the size of some of the aggregates used to make certain types of bituminous paving. In some types of bituminous paving the larger aggregates may be one and one half inches in diameter and larger in size. The washer generally is two inches in diameter. If the rod is pushed into the bituminous paving when the mat depth exceeds the desired depth, the washer may be held up by one of these large aggregates before the rod reaches the bottom of the paving layer. This can lead to an inaccurate reading witch results in a false measurement of the thickness of the mat.
When the washer hits a large aggregate and fails to reach the bottom of the pavement layer and an inaccurate measurement is taken. In one situation, the spreader operator may mistakenly believe the depth to be proper and continues to lay down a mat too thick. Or, possibly in another situation, the operator may receive an indication of the depth being too thick. However, since the washer was held up by a large aggregate, the operator would correct for a paving layer having a thickness of too great of a depth by the measured amount. Unfortunately, a correction was made but not great enough because the depth gauge did not measure the true depth. This would result in the amount of reduction to be too small and result in a loss because of the error in the measurement obtained by the rod and washer device. In both of these cases, too much bituminous is being laid and costs can increase by as much as twenty percent.